1. Field of the Invention
The invention relates to a process for the determination of a PDM-induced outage probability of an optical transmission system and to devices and systems to implement the process.
2. Description of the Related Art
Polarization mode dispersion (PMD) is a well known property of optical elements, such as, for example, fiber optics, optical multiplexers and optical amplifiers.
An important cause of polarization mode dispersion rests on the phenomenon that upon launching an optical signal in an optical element generally several so-called polarization modes are initiated which propagate at different speeds. An initiation of several modes of polarization may, for example, also be the consequezce of a change of the refractive index into a single direction as a response to mechanical irregularities, for example, by curvatures, pressure, tension and/or temperature, of an optical element by means of which one of the orthogonal polarization modes is transmitted faster than the others. In total, the different speeds of several polarization modes lead at the output of the optical element to a distortion of the optical signal and are, therefore, the cause of signal degradation.
Now if in an optical transmission system several optical elements are cascaded then accordingly the PMD-induced signal distortions caused within the individual optical elements shall be accumulated. Provided that these distortions of the signal remain within certain tolerance limits, the optical transmission system fundamentally still works within an admissible area.
For example, for a digital optical signal the bit error rate (BER) constitutes a typical measurement of admissible signal changes, whereby above a certain bit rate threshold value (BTH) the resulting signal degradation reaches such a proportion that the optical signal transmission has to be classified as flawed in its totality.
Processes for measuring the polarization mode dispersion of individual optical elements are known, for example, from the recommendation “ITU-T-G.650.2”, wherein ITU-T stands for a specific committee within the international body “International Telecommunication Union” which devotes itself to standardization in the area of communication engineering and issues corresponding recommendations.
Moreover, there are fundamental models available which state the probability of the occurrence of Differential Group Delays (DGD) in so-called DGD values if the polarizations mode dispersion values have been previously measured.
Publications to be named in this context are, for example, the ITU-T recommendation No. G.691 and the Technical Report (TR) IEC/TR 61282-3 which were issued by the International Electrotechnical Commission (IEC). What should be considered disadvantageous in this context that by making use of the differential group delays it is always only possible to present a part of the effects which are based on polarization mode dispersion.